1. Field of the Invention
The invention relates to a method for magnetic resonance imaging of a density distribution of a first substance and a second substance in a part of a body which is arranged in a substantially uniform, steady magnetic field, the first substance having a J-coupling, which method includes the following steps:
a) exciting spins of the first substance and the second substance in the part of the body by means of an excitation pulse, PA1 b) generating pulse sequences for measurement of MR signals which pulse sequences include a series of successive refocusing pulses and first and second gradients in mutually orthogonal first and second directions, respectively, the repetition time between the refocusing pulses in said pulse sequences being substantially equal to the reciprocal value of the J-coupling, PA1 c) fitting measured values of the MR signals in a 3-dimensional data matrix, a first and a second dimension of which correspond to the respective gradients in the first direction and the second direction whereas a third dimension corresponds to a measuring time of an MR signal, and PA1 d) determining a density distribution of the first substance or the second substance in the part of the body from the data matrix by means of Fourier transformations. PA1 a) exciting spins of the first substance and the second substance in the part of the body by means of an excitation pulse, PA1 b) generating pulse sequences for measurement of MR signals, which include a series of successive refocusing pulses and first and second gradients in mutually orthogonal first and second directions, respectively, the repetition time between the refocusing pulses in said pulse sequences being substantially equal to the reciprocal value of the J-coupling, PA1 c) fitting measured values of the MR signals in a 3-dimensional data matrix, a first and a second dimension of which correspond to the respective gradients in the first direction and the second direction whereas a third dimension corresponds to the measuring time of an MR signal, PA1 d) determining a density distribution of the first substance or the second substance in the part of the body from the data matrix by means of Fourier transformations, the control unit also being arranged to apply the first gradient in such a manner that: in the first dimension of the data matrix a value of the MR signal measured after a refocusing pulse bearing an even sequence number in the series of successive refocusing pulses succeeds a value of the MR signal measured after a refocusing pulse bearing an odd sequence number, a distance between two adjacently situated grid points in a k.sub.x direction, corresponding to the first direction, being equal to ##EQU3## where FOV represents the field of view in the first direction.
The invention also relates to an MR device for executing this method.
2. Description of Related Art
In the context of the present invention a gradient is to be understood to mean a magnetic field gradient pulse which superposed on the magnetic field yields a magnetic field gradient in a predetermined direction. J-coupling is to be understood to be a spin-spin coupling between a spin state of a first nucleus and a spin state of a second nucleus which is situated near the first nucleus and exhibits a chemical shift which differs from that of the first nucleus. This J-coupling causes a division of a spectral line in the MR spectrum. Generally speaking, this division is small with respect to the chemical shift between the nuclei. Furthermore, the J-coupling is independent of the applied magnetic field.
A method of this kind is known from International patent application WO 95/04610. The known method is used for the spectroscopic imaging of parts of the body so as to study the pathology of the parts imaged with a view to determine whether the pathology is a tumor, an abscess or a lipoma. A lipoma is a fat-filled space in biological tissue. Substances which play a part in this study are substances exhibiting a J-coupling, such as inter alia lactate, and substances which do not exhibit a J-coupling, for example lipids.
The known method utilizes the pulse sequences so as to acquire MR signals containing spatial and spectroscopic information. To this end, the spins within the object are excited and subsequently spatially encoded by the first and second gradients which are applied during the preparation period. The application of the refocusing pulses gives rise to an MR signal. By choosing the repetition time between the refocusing pulses to be equal to the reciprocal value of the J-coupling, a 180.degree. phase modulation occurs in the contribution of the spins of the first substance to the MR signals succeeding the refocusing pulses. After a phase correction and an amplitude correction, the contribution to the MR spectrum by the second substance is reduced by linear combination of the MR signals received. An image of the density distribution of the first substance in the part of the object is determined from the processed MR signals by means of a three-dimensional Fourier transformation. It is a drawback of the known method that the density distribution of the first substance is contaminated by MR spectroscopic information of the second substance.